1. Field of the Invention
The present invention relates to an elastic endoscope having an insertion portion made of a flexible tube which is to be inserted into a body tube or a body cavity.
2. Description of the Related Art
Elastic endoscopes each having an insertion portion made of a flexible tube which is to be inserted into a body tube or a body cavity are disclosed, for example, in Published Unexamined Japanese Patent Application (PUJPA) No. 2-25816, Published Unexamined Japanese Utility Model Application (PUJUMA) No. 63-180003, and PUJPA No. 61-277915. In these prior-art endoscopes, an outer-sheath tube of the insertion portion is made of a viscoelastic material such as synthetic resin. Most of the characteristics of the insertion portion, such as flexibility and resiliency are determined by the characteristics of the synthetic resin itself of the outer-sheath tube.
The conditions of the outer-sheath tube of the insertion portion of the endoscope, e.g. softness of the outer-sheath tube, are determined in accordance with the state of a region-of-interest into which the end-oscope is inserted. A synthetic resin material having characteristics which meet the conditions is selected for each type of endoscope and is used as a material of its outer-sheath tube. For example, a relatively soft synthetic resin material is selected and used as material of the outer-sheath tube of the insertion portion of the endoscope to be inserted into a region-of-interest which requires softness of the outer-sheath tube. On the other hand, a relatively hard synthetic resin material is selected and used as material of the outer-sheath tube of the insertion portion of the endoscope to be inserted into a region-of-interest which requires suitable hardness of the outer-sheath tube, in order to achieve smooth insertion and easy operation of the endoscope.
In the meantime, the length of the insertion portion of the endoscope varies depending on the region-of-interest into which the endoscope is inserted. When the length of the insertion portion of the endoscope is relatively large, it is difficult to pack or store the endoscope in the state in which its insertion portion is extended substantially straight. In this case, the endoscope is packed or stored in the state in which the insertion portion is bent in a suitable shape or wound in a substantially annular shape.
However, in the conventional endoscope, the outer-sheath tube of the insertion portion is made of a viscoelastic resin material and has both elasticity and viscosity. Thus, the characteristics of the outer-sheath tube are influenced by temperatures and time, and such problems as creep or stress relaxation phenomenon tend to occur.
Consequently, when the endoscope is packed or stored in the state in which the insertion portion is bent in a suitable shape or wound in a substantially annular shape and the packed or stored endoscope is left for a long time, a stress relaxation phenomenon may occur. As a result, bending of the insertion portion of the endoscope remains even after the endoscope which has been in the packed or stored state for a long time is taken out of this state.
When bending of the insertion portion remains after it is taken out of the packed or stored state, insertion of the endoscope becomes difficult. Thus, smooth insertion and easy operation of the endoscope cannot be achieved.